A torque limiter generally comprises an input member 131a for entering a driving force, and an output shaft 131h, for example, as shown in FIG. 18, and when the load torque that the output shaft 131h receives a specific value, it is so designed that the output shaft 131h may rotate coaxially with the input member 131a.
In this prior art, a shake-proof washer 131b is formed at an end surface of the input member 131a, and a ball 131d held in a ball holding plate 131c formed integrally with an output shaft 131h is pressed to the shake-proof washer 131b with a spring 131f.
As shown in FIG. 18(b), while the ball 131d is pushed into the bottom of the shake-proof washer 131b, the shake-proof washer 131d drives the ball 131d in the rotating direction of the input member 131a, and the torque of the input member 131a is transmitted to the ball holding plate 131c holding the ball 131d and to the output shaft 131h.
When the load is applied to the output shaft 131h, the ball 131d is driven by the load in a reverse direction of the rotating direction of the input member 131a, and moves along the ridge of the shake-proof washer 131b. The moving of the ball 131d along the ridge is suppressed by the pressure of the spring 131f, and as shown in FIG. 18(c), the ball 131d is further moved by the load to the ridge of the shake-proof washer 131b, when the transmission of torque from the input member 131a to the output shaft 131h is cut off, thereby preventing generation of overload.
After interruption of the torque transmission, it is free whether or not to continue to drive the input member 131a, but usually driving of the input member 131a is stopped so as not to consume the energy wastefully.
In such a constitution, when it is demanded that the output shaft 131h should be stopped at a specific rotating position, a positioning mechanism 133 as shown in FIG. 19 is provided. The positioning mechanism possesses a positioning groove 133b at one point of the peripheral edge, and further comprises a rotary plate 133a coupled with the input member 131a and a pawl 133c pressed to the peripheral surface of the rotary plate 133a by a spring 133d, and by fitting the pawl 133c into the groove 133b, the stopping position of the output shaft 131h is determined, and when starting up, by pulling out the pawl 133b from the groove 133b by an actuator not shown through, for example, a lever 142' by overcoming the spring 133d, the positioning action is cleared.
According to the positioning mechanism 133, first, when the pawl 133c is pulled out of the groove 133b by resisting the spring 133d, the positioning action is cleared. Then, when the output shaft 131h is rotated to actuate the torque limiter (or by the manipulation of the operator before the torque limiter is actuated), the action of the actuator is stopped, and the pawl 133c is pressed against the periphery of the rotary plate 133a by the spring 133d. Afterwards, the output shaft 131h rotates, the pawl 133d fits into the groove 133b, and the output shaft 131h is positioned and stopped.
As a result, the load torque applied to the torque limiter through the rotary plate 133a exceeds a specific value and the torque transmission is cut off by the torque limiter, thereby preventing the motor from being overloaded. By the position of the pawl 133c and the rotation phase of the rotary plate 133a, when it is detected that the roller stops at a specific position, the motor is stopped.
It is also possible to actuate the positioning means after starting the torque limiter. That is, after the torque limiter is put in action, it is designed to have the pawl 133c engaged in the groove of the rotary plate 133h which rotates by inertia.
According to the conventional constitution, as shown in FIG. 17(a), when the ball 131d pushed into the bottom of the shake-proof washer 131b rides over the top of the shake-proof washer 131b, the torque applied to the input member 131a increases gradually. When the torque applied to the output shaft 131h is larger than the torque shown in FIG. 17(a), the torque limiter is actuated and hence no problem occurs, but as explained later, when the output of the output shaft 131h is used in twisting a wire rod, the load torque applied to the output shaft 131h increases slowly depending on the twisting amount of the wire rod as shown in FIG. 17(b). In this case, the actuating position of the torque limiter (the rotating position: B-C in FIG. 17(a)) becomes unstable, and the torque limiter often fails to act at an expected position.
Furthermore, in the conventional positioning mechanism, to actuate the torque limiter by causing the pawl 133a to engage with the groove 133b by force before actuation of the torque limiter, the output shaft 131h stops, by the manipulation of the operator, regardless of the load side torque.
However, as explained below, when the twisting torque of the wire rod is demanded to be more than a specific value in the case of, for example, using the torque limiter in twisting of wire rod, the constitution for stopping the output shaft 131h by the operator as described above is useless.
Yet, in the constitution for positioning the output shaft which rotates by inertia after actuation of the torque limiter, a flywheel and others are needed for keeping the inertia of the output shaft 131h, which results in a larger size.
The invention is hence proposed in the light of the conventional circumstances as stated above, and it is an object thereof to present a torque limiter which acts accurately depending on the load torque.
It is another object to present a torque limiter provided with a positioning mechanism in a simple constitution.
It is a further object to present a torque limiter provided with a positioning mechanism capable of stopping at an accurate position.